Tilbury Fort is a hugely impressive structure, and shows how the traditional British castle-fortress evolved to cope with the demands of modern warfare. Built in the late 17th century to the design of Sir Bernard de Gomme, Chief Engineer and Surveyor General of Ordinance to Charles II, Tilbury Fort was sited over an earlier Blockhouse dating from the reign of Henry VIII. With another fort, on the opposite bank of the river at Gravesend, it was intended to provide the first line of defence for London. Even today, Tilbury Fort remains virtually unaltered and survives as probably the best example of its type in the country.

Following a regular pentagon in shape, Tilbury Fort was planned with four massive diamond-shaped bastions at each corner, and a further centrally placed bastion in the southern wall facing the river (this southernmost bastion was, for whatever reason, never built). The four huge bastions projected out from the main curtain walls, and their distinctive shape was designed to allow a spread of covering fire along the adjacent curtain walls, across the moat, and over the outer defences. Both the bastions and the curtain walls that joined them were low to the ground. Covered by turf ramparts, they were not only very strong, but also extremely difficult to see and hit.

The entire landward structure was then completely surrounded by a 'V' shaped moat, the level of which could be controlled by a series of sluices, with the River Thames protecting the southern elevation in a similar fashion. Projecting from the northern curtain wall was a bridge that led to a triangular-shaped island, situated within the inner moat. This was known as a 'Ravelin' and was designed with two main purposes in mind. Firstly, to protect the 'Landport Gate' (Tilbury Fort's northern entrance) from direct fire, and then to provide covering fire to any breach in the northern curtain wall or bastions. From the Ravelin another bridge allowed access to a complex series of covered passages, which extended to almost the entire perimeter of the bastions and curtain. These passages would allow infantrymen, carrying small arms, to fire across an outer moat encircling the entire landward complex. Beyond the Ravelin, but sited as a triangular island within the outer moat, was a Redan. This was connected to the mainland via a causeway and formed the main entrance to Tilbury Fort from the landward side. Originally it would have contained a small Redoubt, but this has since disappeared.

The general principle was that most attacks would be from land, making the northern entrance the most vulnerable point. The defences were necessarily heavier here, and designed so that as each line of defence fell, the garrison could fall back to the next one, until all troops were back in the main fort and the drawbridges over the inner moat could be raised. Internally, Tilbury Fort comprises of a large parade ground some 2.5 acres in size. To the north, either side of the Landport Gate, are two rectangular buildings which served as powder magazines. Built c1716, these have been altered over the years, but one of their most notable features is the large copper doors, now green in colour due to weathering.

Running parallel to the west and east elevations of the fort were two rows of barracks. The eastern 'Officers Barrack' still survives, although much altered over the years, and now forms a fine looking row of terraced houses. The soldier's barracks were less fortunate. Having been severely damaged during a Second World War bombing raid, they were subsequently demolished. Other buildings to the north-east of these barracks, which would have included a mess, hospital and kitchens, also suffered the same fate. To the south-west, the Guardroom with the fort Chapel above, and the beautifully ornate 'Water Gate' are two of the original buildings to survive.

The south-east corner of Tilbury Fort is where the most obvious remodelling work has been undertaken. During the late 18th century on the advice of a military engineer, Thomas Hyde Page, the first programme was put in place, followed by the more substantial works of c1868 that the visitor largely sees today.